Approaches to Long-Term Performance Assessment of Deep Underground Disposal of Radioactive Wastes: A European Perspectiv
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APPROACHES TO LONG-TERM PERFORMANCE ASSESSMENT OF DEEP UNDERGROUND DISPOSAL OF RADIOACTIVE WASTES: A EUROPEAN PERSPECTIVE TREVOR J. SUMERLING* AND DAVID READ** *Safety Assessment Management, 4 Freedom Cottages, South Stoke, Reading RG8 OJD, UK **WS Atkins Science and Technology, Woodcote Grove, Epsom, Surrey KT18 5BW, UK
ABSTRACT The aim of long-term performance assessment (PA) is generally to develop confidence that the disposal system will perform within certain bounds and give rise to acceptably low impacts or risks. The PA procedure must be able to deal with: - uncertainty due to incomplete understanding of the relevant processes; - uncertainty due to limited sampling of the structures and characteristics of the system and especially the geological media; - uncertainty about future conditions and processes leading to changes in the engineered barriers and natural environment that may affect radionuclide release, transport and exposure pathways. A number of approaches have been taken to solving the problem of how to deal with such uncertainties. The differences in approach may be due to the stage of project development, differences in national regulatory guidance and the different r6les of the implementor and the regulator. This paper examines the r~les of the implementor and regulator and discusses possible approaches appropriate to both points of view.
INTRODUCTION In this paper, it is accepted that performance assessment (PA) methods based on calculations of possible performance and radionuclide releases, coupled with sufficient information from proposed disposal sites, can provide the technical basis to decide whether specific disposal systems would offer a satisfactory level of safety to society [1]. PA together with controlled experiments and observations on natural geological systems, engineered barriers and waste materials, are designed to investigate the potential behaviour of disposal systems and also, ultimately, to generate confidence that a waste disposal system will not give rise to unacceptable risks over protracted periods of time. PA studies must include a formalised treatment of uncertainty arising from interalia: -
incomplete understanding of the disposal system and the relevant processes, limited sampling of the inherently variable natural systems, the need to extrapolate mathematical models of radionuclide release, transport and exposure processes over spatial and temporal scales beyond those for which they can be tested against real data, the uncertain evolution of the engineered barriers and natural environment as affected by geological, climatic and human factors.
Several organisations now invoke highly evolved PA methodologies to make estimates of possible performance accounting for such uncertainties. The procedures followed vary not only from country to country but also depend on whether the organisation concerned is actually responsible for waste disposal (the implementor) or is responsible for authorization of waste disposals (the regulator). The requirements of the two r6les are
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